1. Technical Field
This disclosure is related to microwave plasma systems and, more particularly, to a fluid-cooled microwave plasma applicator for producing reactive gaseous species for processing applications.
2. Discussion of Related Art
Reactive gases and gas mixtures are used in many industrial operations including the processing of materials such as semiconductor wafers for fabricating electronic and optical devices. Reactive gases can be used, for example, in thin film deposition and etching in microelectronics manufacturing to etch dielectric and semiconductor materials or various masking films such as photoresist and polyimide. Reactive gasses can be used to form dielectric films and metal films, and can also be used to clean wafer surfaces in various stages of wafer processing.
Reactive species of gas molecules can be produced by exciting gas molecules in a plasma discharge. The discharge can be created with a plasma source by coupling energy into a plasma discharge tube or a dielectric window on a chamber containing the gas. Microwave energy is often used as the energy source to create and sustain a plasma discharge. A typical microwave frequency used for creating plasma discharges is 2.45 GHz, due to the availability of power sources and system components.
It is desirable to have a plasma source which is capable of producing a large quantity of various reactive gaseous species under very clean conditions. Examples of desirable species include the various atomic halogens (atomic fluorine, chlorine, bromine, etc.), atomic oxygen, atomic nitrogen, and atomic hydrogen. One technical difficulty in using microwave energy for creating a large quantity of reactive gaseous species in a plasma source is cooling the plasma discharge tube or dielectric window. Air cooling can be used for the plasma discharge tube, but it is relatively inefficient compared with liquid cooling. In addition, air cooling requires relatively large and expensive air blowers or compressors to remove a sufficient amount of heat. Also, air cooling may not be compatible with modern clean room environments used for manufacturing semiconductors.
Liquid cooling is advantageous because it is efficient. Water cooling is particularly desirable because water has high heat capacity, and it is both safe to handle and environmentally benign. Also, chilled water is readily available in nearly all manufacturing, university and research and development facilities. A barrier to using water for cooling microwave plasma discharge tubes is that water also readily absorbs microwave energy. Similarly, many other desirable cooling liquids readily absorb microwave energy.
Certain fluids such as silicone oils, some chlorofluorocarbons, and various hydrocarbon compounds do not absorb microwave energy and thus can be used to cool the outside of a plasma discharge tube. Unfortunately, these fluids are often environmentally undesirable, hazardous to handle, and expensive. In addition, using these fluids requires the use of closed-loop heat exchangers, which further increases the cost and complexity of the system.
A number of techniques have been used to generate plasmas and to produce activated gases. These include capacitively coupled discharges, inductively coupled discharges and microwave discharges. None of the prior devices have been adequate for producing high-flow-rate, contamination-free, chemically activated gases useful for industrial applications.